fix: Use default Cost Center of the Company for additional discount (#37234)

fix: Set cost center as default company cost center

When Discount Accounting in enabled in Selling Settings, use Company default Cost Center while making GL entries for additional_discount_account

(cherry picked from commit 4ada5a488e)

erpnext/controllers/accounts_controller.py

@@ -1406,7 +1406,7 @@ class AccountsController(TransactionBase):
 						"account": self.additional_discount_account,
 						"against": supplier_or_customer,
 						dr_or_cr: self.base_discount_amount,
-						"cost_center": self.cost_center,
+						"cost_center": self.cost_center or erpnext.get_default_cost_center(self.company),
 					},
 					item=self,
 				)